For vehicles using electric motors and power electronic inverters, two-phase cooling with the coolant changed from the liquid phase to the vapor phase is far more effective than using single-phase such as liquid to liquid heat transfer. The significant latent heat associated with the two phase heat transfer is the reason for making two-phase cooling attractive. This type of cooling addresses the need for increased power density and associated higher heat fluxes in inverters and traction drive motors.
There are two cooling liquids available in an internal combustion engine vehicle. One is the 105° C. ethylene glycol/water supply obtained from the radiator. The other one is the 85° C. transmission oil. Strictly speaking, there is also refrigerant at high pressure available for passenger compartment air conditioning. Most people with the experience of their expensive household air-conditioning bills would have an impression that cooling the motors and inverters with a technology that is related to an air conditioning system would be impractical and expensive. This invention teaches that such a negative impression is not necessarily true when the floating-loop system is used to cool the motors and inverters in conjunction with the air-conditioning system of a vehicle.
The cooling of various loads in a vehicle is currently conducted in a piece-meal fashion. Separate cooling systems are used for the interior, the motor, the electronic devices, etc. To date, innovations in the thermal management of semiconductor devices utilized in power electronics have been relatively limited. Devices dissipating large quantities of heat have traditionally been restricted to passive cooling techniques, channel cooling, solid heat sinks, or fans. With the advent of larger, faster, higher current semiconductors efficient cooling methods are required to dissipate ever increasing densities of waste heat. It is estimated that 55% of electronic product failures are due to excessive temperatures. Universities and industry are currently working to develop new methods to provide thermal management to circuit board assemblies as well as individual silicon dies. Promising new technologies being examined include immersion, jet impingement, and spray cooling. Dielectric fluids with high heat capacities and advantageous electrical characteristics are being investigated to work with these new “wet” technologies. New thermosyphon cooling techniques are also beginning being applied to electronics at both board and individual chip levels.
A method of cooling silicon chips is being researched at Purdue University wherein semiconductors are immersed in a dielectric which vaporizes as the chips increase in temperature. The vapor condenses as it rises and is cooled by the water pipe, changes phase back to a liquid, and drops back into the pool. The temperature difference between the vapor and the liquid is negligible. For a lower vapor temperature, the water-cooled heat exchanger for a given heat extracted from the multichip modules is comparatively large.
Semiconductors are also being cooled through jet impingement. This technique, as well as spray cooling, is presently being investigated by multiple universities and industrial sources. In both cases the liquid is typically vaporized, cooled, returned to a liquid state and then recirculated.
A thermosyphon assembly developed by Hewlett Packard is utilized to implement a two-phase liquid cooling system by indirect contact with electronics. In this system the density difference between the liquid and vapor creates a pressure head, which drives the flow through the loop, and as such no additional driving force is needed. Hewlett Packard's assembly reportedly dissipates 80 W of heat from the PC processor.
Hewlett Packard has also expanded their inkjet printer technology to thermal management applications using phase change cooling. Using their existing inkjet knowledge-base they are able to precisely target specific areas of chips as well as control flow volume and rates. This technique allows spatial thermal control onto specific regions of the chip according to its heat level.
Research is also being performed in spray cooling semiconductor technologies at UCLA. Tests have been performed on cooling IGBT's with results of up to 34% improvement seen in their power handling capabilities. Water is being utilized as the coolant in these systems with the semiconductors being coated with a conformal dielectric. Additionally, UCLA's technology involves the construction of the nozzle array from silicon by reactive ion etching.
Isothermal Systems Research is developing thermal management of enclosed electronics at small system levels. Their thermal management applications also include the cooling of individual electronic devices.
The Laboratory for Physical Sciences located adjacent to the University of Maryland's College Park Campus, is a facility where university and federal government personnel collaborate on research. Faculty and students from the UMCP Departments of Physics, Electrical and Computer Engineering, and Materials and Nuclear Engineering conduct research at the LPS laboratories in various fields. The Thermal Management group at LPS is currently developing advanced spray cooling techniques for high-performance computing platforms. Their work includes individual chip cooling as well as circuit board and system level cooling.
Other companies are currently developing and marketing immersion and spray cooling thermal management systems. Modine Manufacturing, which acquired Thermacore in 2001, markets a broad range of loop thermosyphon and heat pipe cooling solutions to military and industrial users at both system and board levels. Heat pipe technology consists of a vacuum tight envelope, a wick structure and a working fluid. The heat pipe is evacuated and then back-filled with a small quantity of working fluid, just enough to saturate the wick. The atmosphere inside the heat pipe is set by equilibrium of liquid and vapor. As heat enters at the evaporator, this equilibrium is upset generating vapor at a slightly higher pressure. This higher pressure vapor travels to the condenser end where the slightly lower temperatures cause the vapor to condense giving up its latent heat of vaporization. The condensed fluid is then pumped back to the evaporator by the capillary forces developed in the wick structure. This continuous cycle transfers large quantities of heat with very low thermal gradients. A heat pipe's operation is passive, driven only by the heat that is transferred.
Thermacore is expanding their heat pipe cooling applications by embedding heat pipes into heatsinks for use under power semiconductors. For example heat pipes were embedded under each of eight power amplifier modules. The heat pipes were 0.375″ in diameter flattened into grooves in the heat sink base with a thermal epoxy at the interface. This approach reduced the thermal resistance of the heat sink by 50%. Thermacore's cooling designs specifically geared towards power applications include loop thermosyphons where the circuit board is essentially immersed in the coolant and vapor chambers.
A vapor chamber is a vacuum vessel with a wick structure lining the inside walls that is saturated with a working fluid. As heat is applied, the fluid at that location immediately vaporizes and the vapor rushes to fill the vacuum. Wherever the vapor comes into contact with a cooler wall surface it will condense, releasing its latent heat of vaporization. The condensed fluid returns to the heat source via capillary action, ready to be vaporized again and repeat the cycle. The capillary action of the wick enables the vapor chamber to work in any orientation with respect to gravity. A vapor chamber heat sink consists of a vapor chamber integrated with cooling fins, pins, etc. Due to the way the vapor chamber operates, the heat source can be placed anywhere on the base without affecting its thermal resistance. In addition, there can be multiple heat sources dissipating the same or different amounts of power. The rate of fluid vaporization at each source will stabilize and the vapor chamber will be nearly isothermal. Thermacore is utilizing this technology in the cooling of power semiconductors.
The cooling approaches described above are solving thermal problems in a piece-meal fashion. This invention looks into the cooling and heating of the hybrid, fuel cell based, and full electric vehicles from a system approach. By doing so, individual components in the system may carry multiple functions. This results in a lower cost, smaller volume, and higher efficiency system.